The present invention is related to printing systems incorporating light emitting print bars as the imager, and more particularly, to a printing system using LED arrays in conjunction with novel gradient index lens arrays to increase or decrease resolution of an image in an image plane.
Image print bars used in xerographic recording systems are well known in the art. The print bar generally consists of a linear array of a plurality of discrete light emitting diodes (LEDs) or light reflecting (liquid crystal) elements. Light emitting diode (LED) arrays are preferred for many recording applications. In order to achieve high resolution (spi), a large number of light emitting diodes, or pixels, are arranged in an LED array and means are included for providing a relative movement between the array and the photoreceptor so as to produce a scanning movement of the array over the surface of the photoreceptor. Thus, the photoreceptor may be exposed to provide a desired image one line at a time as the LED array is advanced, relative to the photoreceptor, either continuously or in stepping motion. Each LED in the array is used to expose a corresponding pixel in the photoreceptor to a value determined by image-defining video data information applied to the print bar.
FIG. 1 shows a prior art printing system which includes an LED print bar 10, consisting of an LED array 12 and a gradient index lens array 14 (the array being sold under the name SELFOC.TM., a trademark of Nippon Sheet Glass Co. LTD.). Array 12 is selectively addressed by video image signals processed through a control circuit (not shown) to produce a modulated resolution output which is coupled through lens array 14 onto the surface of previously charged photoreceptor belt 16. Upstream of the exposure station is a charge device 18 which places a predetermined charge on the surface of belt 16. As the belt moves in the indicated process direction, array 12 is addressed to provide an exposure pattern in response to the video data input. The exposure pattern begins when the leading edge of the image area 20 reaches a transverse start of exposure line represented by a dashed arrow 22. The exposure pattern is formed of a plurality of closely spaced transverse scan lines 24 shown with exaggerated longitudinal spacing on image area 20. Downstream from the print bar 10 location are conventional development, transfer and fusing stations (not shown) which are well known in the art.
LED print bars are available commercially in resolutions of 240 spi, 300 spi, 400 spi and 480 spi. Although 600 spi bars are presently available, they are relatively expensive. For some systems, multiple resolution print bars are required. For example, a given printer may receive input data signals in bit map form at several resolutions. IBM printers operate at 240 and 480 spi standards, Xerox Corporation at 300 and 600 spi standards. Heretofore, multiple resolution print bar requirements have been met by incorporating two or more print bars, each providing the required resolution.
It is therefore an object of the present invention to provide a printing system utilizing image print bars with the capability of achieving 600 spi resolution at low cost.
It is further object to provide a printing system which can increase the given resolution of a print bar.
It is a still further object of the invention to provide a printing system with a plurality of LED print bars, at least one of the LED print bars having a first and second resolution.
It is a further object to improve the connectivity of multiple printer-to-computer configurations.
These objects are realized by utilizing, in a preferred embodiment, a two row, square ended, reduction/enlargement gradient index lens array, the lens array coupling the print bar output to the image plane. The lens array is modified to maintain the radiometric speed needed to provide quality printing images, while also maintaining quality image formation at full field. More particularly, the present invention relates to an image print bar for forming images at a photosensitive surface moving in a process direction comprising:
at least one LED array including at least one row of LEDs positioned adjacent said photosensitive surface and aligned perpendicular to the process direction of said surface,
means for selectively energizing individual LEDs to provide a modulated radiation output at a first characteristic resolution,
a reduction/enlargement gradient index lens array mounted in an optically coupled relationship between said LED array and said photosensitive surface,
the improvement wherein the lens array has a radiometric speed of at least f/3, and
wherein the magnification value of said lens array is set to focus said modulated output onto said photosensitive surface at a second resolution which is different from said first resolution.